The Physics of LASERs - American Physical Society

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Lesson 2.
How Does a LASER Work?
Goal
Students should gain understanding about the fundamental parts of a laser
and how they affect the output of the laser. The fundamental parts of a laser
are:
• atoms or molecules (the active medium)
• an energy source
• a resonant cavity
Time: 60 to 80 minutes – Prior to students working with the computer simulation,
20 to 40 minutes is needed to discuss the parts of a laser and how it
works. An additional 40 minutes is needed for Parts A and B, or these could
be assigned as homework depending on computer availability.
Summary of Lesson 2
Students consider what causes laser light to have its special properties. They
are introduced to the three essential parts of a laser: the active medium (atoms
or molecules that transform the input energy into the output energy),
an energy source, and a resonant cavity. Students revisit the concept of
conservation of energy to support the need for an energy source. Students relate the resonant cavity to collimation.
Students then explore the role the active medium plays. They are introduced to the interaction of light and matter by
absorption, spontaneous emission, and stimulated emission. Through participation in a kinesthetic model, students simulate
the processes of absorption and the two types of emission. Students relate stimulated emission to coherence.
In Part A of this lesson, students explore the fundamentals of a laser using a computer model called the PhET simulator.
Students investigate the laser using a single atom for the active medium, and observe how the three basic parts of the
laser produce laser light.
In Part B, students explore how to construct a multi-atom laser with the PhET simulator in order to understand how the
variables of the laser affect the lasing process. Parts A and B can be completed independently, as a group, or as a class
activity.
Preparation before class
Familiarize yourself with the PhET laser simulator and prepare for the demonstration. Determine if you will have each
group use a computer, a set of groups use a computer, or if the simulations will be teacher-led with the entire class participating.
Decide if students will do Part A and or Part B during class or outside of class.
Materials
• computer projection system
• computer(s) that can run the PhET
simulator with simulator loaded onto
each
• to find the PhET simulator, search on
“PhET Laser Simulator ” or go to http://
phet.colorado.edu/simulations/sims.
php?sim=Lasers
• projections of Lesson 2 images
• masking tape
• sheets of paper (e.g. red construction
paper)
Prepare the room for the kinesthetic models as was done at the end of Lesson 1, for the laser. Two objects will be needed
to represent the mirrors that create the laser cavity. These objects could be walls or desks. The width of the cavity should
be at least 5 students wide. The length of the cavity (distance between two mirrors) should be an integer number of steps,
marked by tape or a pattern on the floor. A reasonable cavity distance for this model is about 10 steps.